An implantable cardioverter defibrillator (ICD) is an example of one type of device which requires a high voltage capacitor in order to discharge an adequate amount of energy for defibrillation. Capacitors employed by these implantable devices must have a size, shape and mass that does not compromise the form factor of the device for implant, while having the capacity to deliver the required energy. One particular type of capacitor employed in these devices includes a tantalum anode and a graphite cathode, which are separated from one another by an electrically insulating porous material, for example, a fluoropolymer layer enclosing the anode.
Tantalum is one of a group of metals known as ‘valve metals’, which form adherent, dielectric, metal-oxide films upon anodic polarization in an electrolyte solution. Tantalum anodes are typically made from a tantalum powder, which may be pressed into a slug that has the appropriate form factor for a particular capacitor to fit within a particular device. The tantalum slug is subsequently sintered, in order to bond the particles of the powder together into a coherent and predominantly solid structure, and then formed, or anodized. The formed metal-oxide film (tantalum pentoxide) preferably extends over an entire surface area of the tantalum slug, which surface area preferably extends into all the cavities between the sintered particles of the slug. The character of the oxide film can greatly influence capacitor performance.
The thickness of the oxide film is approximately proportional to the potential applied across the slug during the formation process, and the potential applied to form the anode is typically greater than a maximum potential that would be applied during operation of the capacitor including the anode. An anode is said to be fully formed when the oxide film has reached a certain thickness and has a certain structure to effectively hold a charge, at the operating potential of the capacitor, for an appropriate amount of time, without allowing an excessive amount of charge to leak out. A charge efficiency of the capacitor can be greatly influenced by the capacity of the anode to hold a maximum amount of charge. Methods/processes have been developed to fabricate tantalum anodes that hold an adequate amount of charge, without excessive current leakage, in order to provide adequate capacitor efficiency. However, there is still a need for new processes/methods to fabricate tantalum anodes that have a capacity to hold more charge without exhibiting significant current leakage, so that an energy density of the anodes (deliverable energy per volume of the anode) is not compromised, and the creation of even more efficient capacitors for high voltage applications in implantable medical devices is facilitated.